The present invention provides an improved nozzle shut-off valve for use in an injection molding machine for plastic. One aspect of the invention relates to an improved nozzle shut-off valve having a pilot passage so located as to assist in opening the shut-off valve with the pressure of plastic from or in the injection unit. An aspect relates to an improved nozzle shut-off valve for use in reciprocating screw or plunger type injection molding machines of the kind used to handle plastic and elastomeric material.
Conventional molding apparatus of the reciprocating rotating screw type usually includes a plasticizing cylinder or chamber having a bore, wherein the plasticizing screw rotates in such a manner so as to allow the solid molding material to enter the cylinder and be plasticized as it advances in the direction of screw feed. Attached on one end of the plasticizing cylinder is a nozzle in communication with a mold sprue which leads to the mold cavity. As the plasticized material is deposited at the metering or front end of the screw, it develops a back pressure that forces the screw to retract in the cylinder bore and when the plasticized material reaches a predetermined volume, or shot size, the retracting screw contacts a limit switch and stops its rotation. The shot is now ready for injection into the mold cavity, generally upon receipt of a signal from the clamp, whereupon the screw is driven forward hydraulically to inject the shot. Later, the plasticizing screw again starts to rotate and gradually retract as a new shot is built up in the plasticizing cylinder. Thus, the screw reciprocates once per machine cycle to plasticize and inject a shot of material.
Often, a shut-off valve is employed to interrupt the flow of molten material from the nozzle into the mold sprue. The valve offers the advantages of minimizing or entirely curtailing drool through cut off of material flow at the nozzle and provide the capability to plasticize during periods in which the mold is open. Generally, plasticizing takes place during part curing to prevent plasticized material from escaping.
Over the years many types of shut-off devices and means to actuate them have been developed and utilized with varying degrees of success. Included among these are, for example, spring operated needle valves such as shown in FIG. 4 of U.S. Pat. No. 3,002,229, which are generally not completely reliable in actual operation; mechanically operated pin valves such as shown in U.S. Pat. No. 3,172,161 which are generally difficult to properly align and have multiple pivot points subject to wear; hydraulically operated rotary valves such as shown in FIG. 3, of U.S. Pat. No. 3,169,275 which are often subject to scoring and difficult to locate near the nozzle because of high nozzle temperatures which make hydraulic operation hazardous; and hydraulically operated piston valves such as shown in FIG. 3 of U.S. Pat. No. 3,026,567 which again are difficult to locate near the nozzle due to the high nozzle temperatures involved.
A preferred embodiment of the present invention employs as drive means a pneumatic motor, thus eliminating the danger of lubricating and hydraulic liquids in this high temperature environment.
Substantial force is necessary to open a nozzle shut-off valve preparatory to an injection cycle. Prior art devices provided this by various arrangements of hydraulic motor, pneumatic piston and cylinder arrangements, and the location and orientation of the several parts.
The present invention facilitates opening and reduces the size of the fluid motor used to open same through the agency of a carefully located pilot passageway that allows the pressure of the plastic itself to open the valve preparatory to an injection operation. The preferred embodiment so locates the passage that it is self-cleaning because plastic can flow through it from one end to the other.
Thus, the present invention contemplates a nozzle shut-off device for use in combination with the nozzle of an injection unit of a plastic injection molding machine, the nozzle having therethrough a substantially straight axial first passageway that is substantially coaxial with the exit passage from said injection unit, which device comprises a body member for interposing between said nozzle and said injection unit and having therethrough a shut-off passageway that is substantially straight and substantially coaxial with said first passageway; a lateral bore in said body intersecting said shut-off passageway and defining a pilot chamber on one side of said shut-off passageway; a plunger reciprocably mounted in said bore and of a cross-section to substantially completely block the following: said shut-off passageway when placed thereacross, said lateral bore, and said pilot chamber, and having a length sufficient when advanced fully to extend across said shut-off passageway into said pilot chamber thereby to block the shut-off passageway; and when fully withdrawn to substantially unblock said shut-off passage; a pilot passage formed in said body and establishing communication between a location under the end of said plunger when same is advanced into said pilot chamber and upstream of the intersection of said bore with said shut-off passageway.
Many prior art constructions present alignment and seal problems because the shut-off valve element is connected to the drive means and both have to be aligned simultaneously during assembly. The present invention eliminates these categories of problems by providing a floating engagement between the valve element (herein a plunger) and the drive means that works when either is pushed into engagement with the other but does not resist pulling the plunger and drive apart.